Sheets of material are often used in various industries and in a variety of ways. These materials can include paper, plastic, and other materials manufactured or processed in webs or sheets. As a particular example, long sheets of paper or other materials can be manufactured and collected in reels.
It is often necessary or desirable to measure one or more properties of a sheet of material as the sheet is being manufactured or processed. For example, in a paper sheet-making process, it is often desirable to measure properties of the sheet (such as its color, gloss, or haze) to verify whether the sheet is within certain specifications. Adjustments can then be made to the sheet-making process to ensure the sheet properties are within the desired range(s).
Many optical and image-based measurements involving a sheet often require the sheet to be confined in a specific position or plane. For example, there is often a narrow range of working distances (from a sensor to the sheet) and/or a narrow range of tilt angles (with respect to illumination or examination of the sheet) that provide proper measurements of the sheet. Deviations from the expected or required distances, angles, or other geometries may introduce bias, uncertainty, or other errors in the measurements. This problem becomes more pronounced when taking measurements of a moving sheet, which may flutter or otherwise move as it passes by or between sensors.
Existing solutions for constraining sheet geometries are often of limited use. For example, solutions that stabilize a sheet for one sensor may disturb the sheet near other sensors. As another example, contacting solutions actually touch the sheet, which can apply friction to the sheet. This may create markings on the sheet, increase the risk of a sheet break, and create difficulties in setting up the contacting solutions. As yet another example, aerodynamic devices often do not guarantee good sheet position or sheet planarity since the sheet's position may be unstable in time and can vary with sheet tension.